This invention relates to improvements in electric power assisted steering systems, and in particular to improvements in the arrangement of electrical circuitry in such systems.
Electric power assisted steering systems are well known in the art. A typical system comprises a first portion of shaft operatively connected to a steering wheel and a second portion of shaft operatively connected to a road wheel of the vehicle through, for example, a steering rack. A torque sensor provides an indication of the torque in the first portion of shaft, and produces an output signal dependent on that torque which is passed to an electrical circuit. The electrical circuit processes the output signal to produce a motor drive signal for a motor which is connected through a gearbox to the second portion of shaft. The motor drive signal corresponds to an assistance torque to be applied by the motor to the second portion of shaft to assist the driver of the vehicle, thus making the steering wheel easier to turn.
The electrical circuit comprises low power devices which modify the output signal from the torque sensor to produce the motor drive signal. This may typically comprise an electronic control unit comprising an integrated circuit or discrete components. The motor drive signal is then further processed by the circuit to produce one or more motor voltage current signals indicate of the current required in each phase of the electric motor. For a typical three phase motor, a pulse width modulation strategy can be employed to drive the motor, whereby the motor drive signal is converted into three motor phase voltage signals which control suitable high power switching circuitry to apply the appropriate voltage waveform to each motor phase. A capacitor may also be provided which is placed in the motor current path to enable a measurement of motor current to be made. The electronic and electrical circuitry can therefore be considered to be loosely divided into two stages. A first (control) stage comprises the low power components used to modify the torque sensor output signal and generate the appropriate motor drive signal (s). The second stage (power stage) comprises the high power electrical components and associated electrical conductors of the electrical switching stage, typically a bridge circuit of high power semiconductor switching devices, for example transistors, and the motor current sense resistor.
It is known in the art in electric power assisted steering systems of the kind set forth to provide both the control circuit and the power stage on a single printed circuit board.
We are also aware of U.S. Pat. No. 5,602,451 in which an electric power assisted steering system is disclosed having both low power components and high power components located on separate printed circuit boards.
In accordance with a first aspect, the invention provides an electric power assisted steering system of the kind which comprises a first portion adapted to be operatively connected to a steering wheel of a vehicle, a second portion adapted to be operatively connected to a road wheel, a torque sensor adapted to provide an output signal indicative of the torque in the first portion, and an electrical circuit adapted to process the output signal from the torque sensor to produce a drive signal for a motor connected through a gearbox to the second portion. in which the electrical circuit comprises a power stage which comprises at least a first conducting means and a control stage comprising a second separate conducting means adapted to carry one or more electrical components of a control circuit, each conducting means comprising one or more conductive tracks or links for carrying current to and/or from components of the electric power assisted steering system, and in which an electromagnetic shield is provided between the first and second conducting means.
Preferably, the first and second conducting means are arranged spaced apart one on top of the other, for example with an air gap therebetween.
By providing two separate stages, each comprising a separate conducting means, it is possible to reduce the cost of the control stage compared with a prior art system in which both the power and control components are provided on one conducting means as the power handling capability of the conducting tracks or links between the components of the control stage can be made lower than that of the tracks or links between components of the power stage.
By conducting means we mean a printed circuit board, lead frame or any other structure comprising a number of conducting tracks or connections adapted to link together one or more of the components of the respective stage to form a circuit and supported or joined by a non-electrically conducting material.
In a most preferred arrangement, the conducting track or connections of the power stage conducting means have a higher power rating than those of the control stage carrier means.
For example, the power stage may comprise a number of conducting tracks encapsulated in a non-conducting plastic material, the tracks being adapted to interconnect at least some of the power stage components. The conducting tracks may be provided in the form of multiple layers of conducting material, such as copper tracks. The power rating is increased by increasing the number of layers for each track or the cross-sectional area of each track.
It is most preferred that the conducting tracks and plastics material are selected such that the power stage forms a substantially rigid planar lead frame. The tracks may then be in the form of a fretwork. The lead frame may include more than one piece of fretwork. Each piece or track may be of different metals to suit different soldering or other connecting techniques. The leadframe does not need to support any of the power stage components although it could be adapted to support one or more components, such as a current capacitor.
A portion of one or more of the conducting tracks may be upstanding from the planar frame for interconnecting with tracks or components of the control stage.
In a most preferred arrangement, the first conducting means comprises a single layer fretwork of conducting tracks embedded in a plastics material. The fretwork may be stamped from a sheet of material such as copper, and placed in a mould so that the plastic material can be injection moulded around it. This produces a rigid carrier means onto which various components can be mounted. The power capacity can then be easily changed from one device to another by increasing the cross-sectional area of the fretwork whilst retaining the same mould.
The control stage preferably comprises a printed circuit board defining the second conducting means upon which the control stage components are mounted. Some or all of these low power components may be surface mounted to minimise both.
The printed circuit board most preferably comprises a four layer 1oz circuit board. Of course, more or less layers, and different weights of tracks could be employed within the scope of the invention.
Preferably, an electromagnetic shield is provided between the first and second conducting means. The shield may comprise a conducting screen having holes or other openings through which the upstanding conductors of the power stage can pass to contact the tracks or components in the control stage, or vice versa. The electromagnetic shield provides a barrier to substanitially prevent electromagnetic radiation from one stage interfering with components of the other stage. This is preferably earthed by a low impedance conducting connection or link to a suitable earth point.
One or more of the high power devices comprising part of the power stage may be provided on a separate substrate and electrically connected to the conductors of the first conducting means. This may be by solder joints to portions of the tracks either upstanding or depending from the first conducting means.
The separate substrate may comprise a material having good thermal conducting and electrical isolation properties, such as IMS. The substrate may be directly connected to a portion of a housing of the electrical motor to dissipate heat. Alternatively, it may be directly connected to a portion of the gearbox which is provided between the motor and the first portion of shaft. The motor housing and/or gearbox therefore dissipates heat generated in the power devices without the need for a separate heatsink. Or course, a separate heatsink could be provided as well/or instead if required.
By providing the first conducting means which provides an electrical connection between components of the power stage and a second conducing means (carrying the control stage) one above the other with an electromagnetic shield between them and securing them directing onto the motor or gearbox, a compact assembly is produced which can be made relatively immune to external electromagnetic interference if the shield is directly connected to the motor or gearbox.
The electromagnetic compatibility of the assembly can be further improved by providing fixing means that co-operates with the shield to provide a mechanical connection from the control stage to the motor housing which is earthed.
Preferably, the power stage conducting means is positioned relative the motor so as to minimise the length of the conductive tracks between the motor windings and the power components such as the motor drive stage switching devices. This ensures optimum efficiency and a minimum power dissipation due to impedance of the conducting tracks or links of the carrier means.
In a most preferred arrangement, the power stage includes a motor current sense resistor which is located substantially in the plane of the conducting fretwork of the first conducting means. In this manner, there are no power conducting paths above or below the high power resistor so mutually induced inductance is minimised.
Since the motor current sense resistor is a relatively large component, placing it in the same plane as the fretwork reduces the height of the assembly. Of course, this can also be achieved when the first conducting means comprises a multi-layer circuit board by placing the resistor in the plane of the circuit board.
By being xe2x80x9cin the planexe2x80x9d, we mean that the axial centre of the resistor is substantially in line with a plane passing through the centre of the carrier means. However, any arrangement whereby at least part of the resistor is above and part of the resistor is below a surface of the first conducting means is envisaged.
A magnetic shield material may be provided on the first conducting means which encapsulates the sense resistor on both sides, or perhaps just one side to reduce electromagnetic interference.
The first conducting means may also carry a connector means which enables electrical connection to be made to one or more the conducting tracks or components for testing or remote operation purposes.